Plasmid maintenance can be subdivided into two primary activities, regulated duplication of plasmid molecules and proper segregation of progeny molecules to daughter cells. In enteric plasmids, mutations causing altered regulation of plasmid copy number and incompatibility may be related because copy number mutants exhibit unselected alterations in incompatibility. In nonenteric plasmids, incompatibility mutants are known to have altered segregation properties. It is not known if incompatibility genes also direct plasmid segregation in enteric organisms. In the enteric plasmid F, genes directing replication, segregation and incompatibility map in a small region designated Rep. The Rep region can be cloned by recombinant DNA techniques; however, the precise locations of the genes directing the various plasmid maintenance functions are unknown. Recently, we have discovered ways of isolating and mapping mutations altering regulation of F replication and other ways of dissecting and cloning subfragments of the F Rep fragment. We propose to use these new mutants and cloned plasmid subfragments to map genes determining plasmid maintenance and to analyze the relationship of these genes to incompatibility. Upon completion of this analysis, we will extend the same type of study to the virulence plasmids Co1V and Ent P307 and the tetracycline resistance plasmid R386 because recent studies suggest that F and these plasmids have identical genes for incompatibility but not for replication. Further, we propose to construct lambda F chimeras containing F replication and incompatibility genes to take advantage of the lambda recombinational system in a fine structure analysis of the F genes.